As is known, a telephone subscriber set, comprising essentially of a speech circuit and a bell connected in parallel to each other, is connected to a telephone exchange by means of a two-wire line. Connected in series to the speech circuit, which would be only closed on hooking off, and serially to the bell, is a capacitor operative to DC uncouple the bell from the line.
A telephone subscriber line is supplied a direct voltage from a source, having an alternate voltage source connected serially thereto, which forms the ring signal generator, when the exchange devices control special line connection contacts to close and issue said ring signals to the subscriber.
The ring signals are sent over the line at predetermined intervals. When the hook-off occurs while a ring signal is being sent, a DC component overlaps the alternate ring current over the line.
With today's telephone exchanges, the hook-off condition is detected by circuit means of an electronic type which sense the presence of direct current on the line due to hooking off, even where it overlaps the alternate current of a possible ring signal. Ring signals usually have a frequency in the 16 to 66 Hz range and voltage RMS values in the 50- to 105-Volt range, that is, much higher values than those of telephone speech signals. However, if the signals are not interrupted directly on hooking off, they are converted, by the speech circuit of the telephone subscriber set, to high-intensity acoustical signals which are harmful to the subscriber's hearing and the set itself. Accordingly, such circuit means are designed to timely sense the hook-off condition and issue an interrupt signal for the ring signals at once to the exchange devices.
Implemented by electronic means are also other functions required for proper operation of the telephone network, such as handling the supply of direct current and sending ring signals, switching from two- to four-wire conversion, providing protection from overvoltages, checking the line operability, and so forth. These functions are performed by a circuit assembly which forms an interface between the telephone exchange control devices and the telephone subscriber line. This is commonly referred to as the electronic subscriber link or SLIC (Subscriber Line Interface Circuit). Most of the circuits of the electronic subscriber link are integrated to few, usually two, monolithic IC devices. The generator of ring signals is in some instances incorporated to one of the integrated circuits, and in some other instances, is a discrete component serving, as a rule, all of the subscriber lines to the exchange. This invention can be advantageously applied to the last-named situation.
A circuit assembly of this kind, as designed and practiced by this Applicant, partly in the form of monolithic integrated circuits designated TDB 7711 and TDB 7722, is presented on page 380 of SGS-THOMSON MICROELECTRONICS handbook "Telecom Data Book," June 1989.